1. Field of the Invention
The invention relates to a process for the preparation of hydrothermally consolidated and hydrolytically stable, consolidated proppants having improved mechanical strength.
2. Discussion of Background Information
Binders are very important in particular for binding compact or granular products. In the mineral oil industry, the process of fracturing has proved particularly useful for increasing and stabilizing oil production in oil-containing deposits. For this purpose, an artificial fissure is first produced in the oil-carrying formation around the borehole by means of a highly viscous fluid (fracture fluid). To ensure that this fissure remains open, the highly viscous fluid is provided with so-called proppants, which maintain the fissure as a region of increased porosity and permeability after removal of the pressure which is necessary for producing and maintaining the formation fissure. Proppants are in particular sands and ceramic particles which have diameters of several 100 μm to a few millimeters and are positioned in the rock fissure. As a rule, these proppants have to be consolidated in order to prevent flowback with the oil produced. Binders which initially cure under the conditions of the reservoir developed (high pressure in combination with high temperature, ground water of the reservoir itself and aggressive components in the crude oils and gasses) and are stable in the long term during oil production are required for this purpose.
For efficient use of binders, it is important that the stability under the abovementioned aggressive conditions is maintained for as long as possible, and the binding strength and the porosity must not be significantly reduced. The systems which are mentioned in the prior art and which are virtually all based on organic polymers have a very limited life in this respect.
The consolidation of proppants with suitable binders is difficult particularly when it is intended that the consolidated proppants do not significantly lose porosity in comparison with the proppants without binder. Thus, porous composites can be prepared, for example, using organic polymer binders, but it is found that it is scarcely possible to maintain the original porosity. With reduced use of binder, it is possible to prepare porous systems but, owing to the properties of the organic polymers whereby they swell or go into solution in the presence of organic solvents, such composites are not suitable for many applications, in particular at relatively high temperatures and in an environment of organic liquids.
The use of purely inorganic binders which are obtainable, for example, via the sol-gel process leads to binding where an appropriate porosity is maintained in the proppant, but the bound system is very brittle, friable and insufficiently resistant to mechanical loads, such as shear loads or strong pressure loads.
Furthermore, it is frequently expedient to prepare proppants under the conditions under which they will also subsequently be used. It is therefore frequently necessary to cure the proppants on site after introduction into the fracture under the geological pressure and temperature conditions. In the case of many consolidating agents, this is not possible or is possible with loss of the necessary stabilities to hydrolysis.
The patent application DE 102005002806.3 of the applicant describes a process for the preparation of consolidated proppants in which a hydrolysate or condensate of an organosilane, a hydrolyzable silane and a metal compound is used as the consolidating agent. The use of this organically modified inorganic consolidating agent affords consolidated proppants which exhibit substantially improved stability to hydrolysis and to corrosion under hydrothermal conditions in comparison with the prior art. Since high resistance to the formation of fragmentary structures is required with the use of the proppants, there is still a need for an additional improvement in the mechanical strength of the proppants obtained.
It was therefore an object of the invention to provide a process for the preparation of consolidated proppants which, under the pressure and temperature conditions of the area of use, are stable to hydrolysis and to corrosion and have a relatively high porosity and moreover possess a further improved mechanical strength.